The turnaround from the disaster that was vanilla D3 to Reaper of Souls was amazing. I hope they can replicate that. I'm not sure I have faith at this point though.
Yeah at this point I don't even know if I can really articulate what I want them to add in the next expansion that would really excite me. I can (and have, in this thread and others) pretty lucidly say what aspects of WoD are lacking/disappointing/shitty, but I've never really believed that "make things like the used to be!" is a terribly meaningful stance.
But let me try!
First, the easy stuff: New classes/races are always fun. I would love to see Demonhunter, or Shadowstalker, or Tinker, or whatever else. Ethereal, Arrakoa, whatever. The more the merrier. They won't make or break an expansion if the content is shitty, but they are among the biggest draws a new expansion can have and are the best way of adding a layer of "newness" to it.
Raids: Atmopshere! I know this is subjective, but although WoD raids were good mechanically, but they didn't
feel fun to be in. I don't think I have ever grown tired of a raid faster than I did with BRF, and Highmaul didn't really wow me either. HFC almost feel like an effort to recapture Black Temple, but is still marred by the overall boring WoD aesthetic design. I want raids that feel epic, and have immersive ambient audio and environments that really pop. I don't even want to give specific examples because there really are a lot of them from the game's 10 year history.
Beyond that, I wish there was
something Blizzard could do to just improve the Group Finder/PuG situation. There is so much
potential there to cater to the huge amount of people who find LFR mindlessly boring but don't want to commit to a progression raid schedule, so they actually experience real raiding and a sense of progression and gear upgrades. However, there is very little incentive to use it that way, and the community has grown so toxic and impatient that you literally have to outgear a raid in order to expect an invite. It's a really shitty situation that I don't know how to fix, but I'm not a game developer. I guess this just ties into the overall social issues of the game at this point, and how rare it is to randomly encounter someone that you end up adding to your friends list and playing with more.
PvP: racial ability balance, obviously, but that might just be a band-aid on a severed head at this point. All things being equal, why would all the people who paid to transfer to alliance to PvP pay to transfer back? Especially with Mercenary mode, there is even less incentive to actually switch back since queue times will be alleviated. I am actually ok with how PvP itemization worked in WoD, so no big complaints there considering the next expansion obviously won't have Ashran shoehorned in.
Non-Raid endgame: Meaningful reputations that aren't just grinds. A reasonable amount of dailies that you can ignore if they don't interest you without feeling like you are gimping yourself. Hopefully they keep Mythic 5-mans as a competitive alternative/supplement to LFR, because more reasons to run 5-mans is always good. They were the bread and butter endgame for a lot of people in BC and Wrath, and the recent trend of having them be a minor stepping stone and then forgotten a month after an expansion releases is really a terrible waste.
Also related to 5-mans: Justice/valor were showing their age, but Apexis were a step backwards. Heroic (and Mythic) 5-mans should definitely contain some sort of currency to keep them at least somewhat relevant as a gearing path/catch-up mechanic.
Show me the goods tomorrow, Blizzard. "we are responding to feed back, plz b excite" is meaningless PR until you actually deliver.